Geotech3 LS9 Piles

7/30/2019 Geotech3 LS9 Piles

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UNIVERSITY OF ADELAIDE

SCHOOL OF CIVIL AND ENVIRONMENTAL ENGINEERING

GEOTECHNICAL ENGINEERING DESIGN III

M. B. Jaksa

PILE FOUNDATION ANALYSIS AND DESIGN

References: Bowles, J. E. (1996). Foundation Analysis and Design, 5th ed., McGraw Hill, 1175p.

Bustamante, M. and Gianeselli, L. (1982). Pile Bearing Capacity Prediction by Means of Static

Penetrometer CPT. Proc. 2nd European Symp. on Penetration Testing, Amsterdam, pp. 493500.

Coduto, D. P. (1994). Foundation Design - Principles and Practices, Prentice Hall, 796p.

Craig, R. F. (2004). Soil Mechanics, 7th ed., Spon Ltd., 464p.

Das, B. M. (1995). Principles of Foundation Engineering, 3rd ed., PWS Publ. Co., 828p.

Fang, H.-Y. (ed.) (1991). Foundation Engineering Handbook, 2nd ed., Chapman and Hall, 923p.

McAnally, P. A. and Douglas, D. J. (1984). The Design of Frankipiles in Clays. Proc. 4th Aust.N.Z. Conf. on Geomechanics, Perth, pp. 402407.

Poulos, H. G. and Davis, E. H. (1980). Pile Foundation Analysis and Design, Wiley, 397p.

Schmertmann, J. H. (1978). Guidelines for the Cone Penetration Test - Performance and Design.

Report No. FHWA-TS-78-209, U.S. Dept. Transportation, Federal Highway Administration,

Washington, 145p.

Tomlinson, M. J. (1986). Foundation Design and Construction, 5th ed., Longman, 842p.

Whitlow, R. (1990). Basic Soil Mechanics, 2nd ed., Longman, 528p.

1. INTRODUCTION

While spread footings are the most common type of foundation, engineers often encounter

situations where deep foundations are more appropriate. Some examples include:

The upper soils are so weak and/or the structural loads are so high that spread footings would betoo large. A useful rule-of-thumb for buildings is that spread footings cease to be economical

when the total plan area of the footings exceeds one-half of the building footprint area.

Expansive or collapsible soils are present at the site of the proposed structure. The upper soils are subject to scour and undermining. The foundation must penetrate through water. A large uplift capacity is required. A large lateral load capacity is required. There will be a future excavation adjacent to the foundation, and this excavation would

undermine shallow footings.

2. AXIAL LOAD CAPACITY OF A SINGLE PILE

The axial load capacity, Pu, of a single, statically loaded pile is shown in Figure 2.1 and is given by:

WPPP busuu += (2.1)

where: Psu is the ultimate shaft resistance;

Pbu is the ultimate base resistance; and


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W is the weight of the pile. In many cases W can be neglected, as it

usually has minimal effect on Pu.

Figure 2.1 Axial load capacity of a single pile.

When Pbu >> Psu the pile is known as an end bearing pile. On the other hand, when Psu >> Pbu the

pile is known as afriction pile.

Due to uncertainties in the soil and, to a lesser extent, the pile material properties, the magnitude of

loading and the mathematical models used to estimate Pu, a lower allowable value, Pall, is the

maximum load appliedto the pile, such that:

WFS

PPP busuall

+= (2.2)

where: FS is the factor of safety against pile bearing failure (usually = 3).

2.1 Load Displacement Behaviour

The pile shaft resistance, Ps, and the pile base resistance, Pb, are not mobilised at the same rate. The

maximum value ofPs (i.e. Psu) is typically reached at displacements of [0.005 0.01] B (whereBis the pile width or diameter). However, much larger displacements are required to fully mobilise

Pb (typically 0.1B for driven piles and perhaps more for bored piles). The relative rates of

development ofPs and Pb are shown in Figure 2.2.

Pu

Psu

Pbu

W


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Figure 2.2 Load displacement behaviour of a single pile.

2.2 Ultimate Shaft Resistance

The ultimate shaft resistance, Psu, can be evaluated by integration of the shear stress along the pile

shaft,fs, over the surface area of the pile shaft, by the following expression:

dzfCP s

L

su =

0

(2.3)

where: C is the perimeter of the pile (circumference of a circular pile);

L is the length of the pile; and

fs is the average shear stress along the shaft of the pile.

The challenge in geotechnical engineering is to determine appropriate values offs. Two of the most

commonly adopted approaches are the method, which is used for piles embedded in clay and silt

soils, and the method, which is used for piles embedded in sand and gravel, as well as clay andsilt. Both methods are highly empirical and they are each treated, in turn, below.

2.2.1 The method (clays and silts)

The method was originally proposed by Tomlinson (1971) and is based on total stresses analysisand undrained shear strengths of cohesive soils and, as a result models short term behaviour. The

average shear stress along the shaft of the pile,fs, is expressed as:

uus scf == (2.4)

where: is the adhesion factor.

Pilehead

load

Pile head displacement

Psu

Pbu

0.005 0.01B 0.1B


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Ideally, the value offs for a given pile at a given site should be determined from a pile-load test, but

since this is not always possible, one must often resort to empirical values, as shown in Figure 2.3.

Bowles (1996) suggested the relationship shown in Figure 2.4.

Figure 2.3 Variation of with cohesion,cu (a) case 1; (b) case 2; (c) case 3.(Source: Tomlinson, 1986.)


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Figure 2.4 Variation of with undrained shear strength,su. (Source: Bowles, 1996.)

2.2.2 The method

The method was proposed by Burland (1973) and is based on an effective stresses analysis and isapplicable to both granular and cohesive soils. The average shear stress along the shaft of the pile,

fs, is expressed as:

vvhs Kf ''tan''tan' === (2.5)

where: 'h is the horizontal effective stress;

' is the effective friction angle between the soil and pile material;

K is the coefficient of lateral earth pressure = vh '' ;

'tan = K

For overconsolidated clays:

( ) 'tan'sin1 5.0 = OCR (2.6)

where: OCR is the overconsolidation ratio.

In most cases, however, it is necessary to determinefs by means ofKand ' separately, as in:

'tan' = vs Kf (2.7)

However, in reality Kis not constant but varies with depth. At the top of the pile it is approximately

equal to the Rankine passive earth pressure coefficient, Kp, and at the pile tip it may be less than the

at-rest value, K0. It also depends on the nature of the pile installation. Table 2.1 gives approximate

average values for Kand Table 2.2 values ofK0 for various relative densities of sand.

Table 2.1 Approximate average values for the coefficient of lateral earth pressure,K.

Pile Type K

Bored K0 = 1 sin

Driven low displacement (Jacked) K0 = 1 sin to 1.4K0 = 1.4(1 sin)

Driven high displacement (Frankipile, screwed) K0 = 1 sin to 1.8K0 = 1.8(1 sin)


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Table 2.2 Approximate values for the coefficient of earth pressure at rest,K0.

Relative Density K0

Loose 0.5

Medium-dense 0.45

Dense 0.35

For overconsolidated sands:

( ) 'sin0 'sin1 OCRK (2.8)

Values of are given in Table 2.3.

The effective vertical stress, 'v, for use in Equation (2.7), increases with pile depth to a maximum

limiting value at about 15 to 20 pile diameters. A conservative approach is to assume that 'v varies

linearly with depth, from zero at the surface, to a maximum value at L' = 15B (whereB is the widthor diameter of the pile).

Based on standard penetration test (SPT) data, the average unit skin friction, fave, is approximately

equal to:

piles)drivenntdisplaceme(Low

piles)drivenntdisplaceme(High2(kPa)

N

Nfave

=

=(2.9)

where: N is the SPT number averaged over the layer in question.

Table 2.3 Approximate values of for various interface conditions (Kulhawy, 1984).

Pile/Soil Interface Condition

Smooth (coated) steel/sand 0.5' 0.7'

Rough (corrugated) steel/sand 0.7' 0.9'

Precast concrete/sand 0.8' 1.0'

Cast in situ concrete/sand 1.0'

Timber/sand 0.8' 0.9'

2.3 Ultimate Base Resistance

It is usually accepted that the ultimate base resistance of a pile, Pbu, can be evaluated from bearing

capacity theory such that:

++= BNNcNAP qvbcbbu 0.5' (2.10)

where: Ab is the area of the base of the pile;

c is the cohesion of the soil beneath the pile base (or su);

'vb is the effective vertical stress in the soil at the level of the pile base;

is the unit weight of the soil;


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B is the width (diameter of a circular pile) of the base of the pile (usually

only used when the base width is enlarged); and

Nc,Nq,N are the bearing capacity factors, introduced previously in the Bearing

Capacity of Shallow Foundations lecture series, as given in Table 2.4.

For a pile founded in a clay and assessed under undrained conditions, = 0 and henceNq = 1 and

N = 0. In this case, a value ofNc = 9 is usually adopted. Equation (2.10) then simplifies to:

( )vbubbu cAP '9 += (2.11)

For a drained analysis (long term assessment) of a pile founded in clay, the drained cohesion, c', is

usually assumed to be equal to zero. Hence, Equation (2.10) simplifies to:

qvbbbuNAP ' (2.12)

because BN0.5 is negligible. The value ofNq depends on the assumed failure mechanism. Janbu

(1976) suggested the following relationship be used:

( )[ ] 'tan225.02 'tan1'tan ++= peNq (2.13)

where: p is the angle ofpastification (the angle between the pile and the plastic

failure zone at the tip of the pile). The angle p varies from /3 for

soft, fine grained soils to 0.58 for dense, coarse-grained soils andoverconsolidated fine grained soils.

Berezantzev et al. (1961) proposed the following relationship for dense sands:

'17.021.0 = eNq (2.14)

Both Janbus and Berezantzevs et al. relationships are summarised in Table 2.4.

2.7 Load Capacity Based on the CPT (LCPC Method)

Bustamante and Gianeselli (1982) proposed a technique, known as the LCPC(Laboratoire Central

des Ponts et Chausses, France)Method, for estimating the axial load capacity of a single pile basedon cone penetration test (CPT) data. It has been shown by several researchers that the LCPC

Method provides the best estimates of any CPT technique in current use.

The LCPC Method is used to predict the ultimate axial capacity of a statically loaded pile, Pu, and is

given by the following equation:

P P Pu b s= + (kN) (2.15)

where: Pb is the resistance due to the base of the pile (kN);

Ps is the resistance due to the shaft of the pile (kN).


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Table 2.4 Values ofNq from Janbu and Berezantzev et al.

Janbu (p = ) Janbu (p = ) Janbu (p = )

/3 0.58

Berez-

antzev

/3 0.58

Berez-

antzev

/3 0.58

Berez-

antzev

0 1.00 1.00 0.21 17 3.46 3.19 3.78 34 14.53 12.09 67.99

1 1.07 1.07 0.25 18 3.74 3.42 4.48 35 15.99 13.22 80.59

2 1.15 1.14 0.30 19 4.04 3.68 5.31 36 17.64 14.47 95.523 1.24 1.22 0.35 20 4.37 3.96 6.29 37 19.50 15.88 113.22

4 1.33 1.31 0.41 21 4.73 4.26 7.46 38 21.59 17.45 134.20

5 1.43 1.40 0.49 22 5.12 4.59 8.84 39 23.96 19.22 159.07

6 1.54 1.49 0.58 23 5.55 4.95 10.48 40 26.66 21.22 188.55

7 1.65 1.60 0.69 24 6.02 5.34 12.42 41 29.74 23.47 223.49

8 1.78 1.71 0.82 25 6.54 5.76 14.72 42 33.25 26.02 264.90

9 1.91 1.83 0.97 26 7.11 6.23 17.45 43 37.29 28.93 313.99

10 2.05 1.96 1.15 27 7.74 6.74 20.68 44 41.94 32.25 372.17

11 2.21 2.10 1.36 28 8.44 7.30 24.52 45 47.33 36.05 441.14

12 2.38 2.25 1.62 29 9.20 7.91 29.06 46 53.59 40.42 522.88

13 2.56 2.41 1.91 30 10.05 8.59 34.44 47 60.90 45.48 619.7714 2.76 2.58 2.27 31 11.00 9.34 40.83 48 69.48 51.35 734.62

15 2.98 2.77 2.69 32 12.05 10.16 48.39 49 79.59 58.19 870.75

16 3.21 2.97 3.19 33 13.22 11.08 57.36 50 91.59 66.21 1032.1

For a multi-layered soil, Bustamante and Gianeselli (1982) suggested that Pb and Ps may be

determined from the following relationships:

P q k Ab ca c p= (kN) (2.16)

where: qca is the clipped average cone tip resistance at the level of the pile base

(kPa);

kc is thepenetrometer bearing capacity factor(Table 2.5);

Ap is the area of the base of the pile (m2).

and:

P q C t s si p ii

n

==

(kN)1

(2.17)

where: qsi is the limit unit skin friction of the ith layer (kPa);

Cp is the circumference of the pile shaft (m);

ti is the thickness of the ith layer (m).

Bustamante and Gianeselli (1982) suggested that the limit unit skin friction, qsi, may be determined

from the following equation:

qq

si

c=

(2.18)

where: is a constant which allows for the nature of the soil and the pile

construction and placement techniques (Table 2.6). Note that thisparameter is not related to the angle of pastification introduced earlier.


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Table 2.5 Values of penetrometer bearing capacity factor,kc .

qc kcType of Soil

(MPa) I II

Soft clay and mud < 0.1 0.4 0.5

Moderately compact clay 0.1 0.5 0.35 0.45Silt and loose sand 0.5 0.4 0.5

Compact to stiff clay and compact silt > 0.5 0.45 0.55

Soft chalk 0.5 0.2 0.3

Moderately compact sand and gravel 0.5 0.12 0.4 0.5

Weathered to fragmented chalk > 0.5 0.2 0.4

Compact to very compact sand and gravel > 0.12 0.3 0.4

Note: Groups I and II are defined in Table 2.8.

Table 2.6 Values of.

qc Type of Soil

(MPa) IA IB IIA IIB

Soft clay and mud < 0.1 30 30 30 30

Moderately compact clay 0.1 0.5 40 80 40 80

Silt and loose sand 0.5 60 150 60 120

Compact to stiff clay and compact silt > 0.5 60 120 60 120

Soft chalk 0.5 100 120 100 120

Moderately compact sand and gravel 0.5 0.12 100 200 100 200

Weathered to fragmented chalk > 0.5 60 80 60 80Compact to very compact sand and gravel > 0.12 150 300 150 200

Note: Groups IA, IB, IIA, IIB, IIIA and IIIB are defined in Table 2.8.

Bustamante and Gianeselli (1982) suggested maximum values for qsi , to account for: the presence

of localised hard elements; non-compliance with standard penetration rates; poor condition of

cones; excess porewater pressures; and deviation of the CPT rods from the vertical. These are given

in Table 2.7. Note that the values in parentheses are where the pile construction involves very

careful and minimal disturbance to the soil, which results in optimal pile-soil friction values.

The clipped average cone tip resistance, qca, is calculated using the following procedure:

1. As shown in Figure 2.5, the intermediate parameter, qca' , is determined by averaging themeasured values ofqc over the length,Lpap toLp +ap, where:Lp is the length of the pile; and ap

is equal to 1.5 B (whereB is the width of a pile, or in the case of a circular cross-section pile, its

diameter).

2. The measured values ofqc are then clipped to remove local irregularities, such that qc is in therange: 0.7 1.3q q qca c ca' ' .

3. The clipped average cone tip resistance, qca, is then determined by averaging the clippedvaluesofqc, over the length,Lpap toLp + ap.


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Table 2.7 Maximum values of the limit unit skin friction, qsi (kPa).

qc qsi (kPa)Type of Soil(MPa) IA IB IIA IIB IIIA IIIB

Soft clay and mud < 0.1 15 15 15 15 35

Moderately compact clay 0.1 0.5 35(80) 35(80) 35(80) 35 80 120

Silt and loose sand 0.5 35 35 35 35 80

Compact to stiff clay and

compact silt

> 0.5 35

(80)

35

(80)

35

(80)

35 80 200

Soft chalk 0.5 35 35 35 35 80

Moderately compact sand

and gravel0.5 0.12 80

(120)

35

(80)

80

(120)

80 120 200

Weathered to fragmented

chalk

> 0.5 120

(150)

80

(120)

120

(150)

120 150 200

Compact to very compact

sand and gravel

> 0.12 120

(150)

80

(120)

120

(150)

120 150 200

Note: Groups IA, IB, IIA, IIB, IIIA and IIIB are defined in Table 2.8. The values in parentheses are relevant to pile

construction involving minimal disturbance to the soil in contact with the pile shaft.

ap = 1.5 Bq

cmeasurements

qca

1.3q'ca

q'ca

ap

ap

0.7q'ca

q'ca

B

Figure 2.5 The procedure used to calculate qca

.

(After Bustamante and Gianeselli, 1982).

Finally, Bustamante and Gianeselli (1982) recommended that the allowable design axial load, Pall,

that can safely be placed on the pile, is given by the following equation:

PP P

allb s= +

3 2(kN) (2.19)

It can be seen from the preceding treatment that the LCPC Method makes use of only the qc

measurements; that is, sleeve friction measurements are not used in the prediction of the static axialload capacity of the pile. This is an advantage over other methods, since this reduces the

computational effort required, as well as the uncertainty associated with the prediction itself.


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Table 2.8 Definitions of group categories.

Pile Type and Construction Method kc qsi and

Plain bored piles I IA

Mud bored piles I IA

Cased bored piles I IB

Hollow auger bored piles I IA

Piers I IA

Barrettes I IA

Cast screwed piles II IA

Driven precast piles II IIA

Prestressed tubular piles II IIA

Jacked metal piles II IIB

Driven grouted piles II IIIA

Driven metal piles II IIB

Driven rammed piles II IIIAJacked concrete piles II IIA

High pressure grouted piles of large diameter (> 250 mm) II IIIB

3. SETTLEMENT OF A SINGLE PILE

The settlement, s, of the top of a pile may be expressed, to sufficient accuracy, in terms of the

settlement of an incompressible pile in a half-space, with correction factors for the effects of pile

compressibility, and so on. The following method is given by Poulos and Davis (1980).

3.1 Floating Pile

dE

PIs

s

= (3.1)

where: s is the settlement of the pile head;

P is the applied axial load;

I = RRRI hK0 (influence factor for a rigid pile in a deep layer);

I0 is the settlement-influence factor for an incompressible pile in a semi-infinite mass, fors = 0.5 (Fig. 3.1);RK is the correction factor for pile compressibility (Fig. 3.2);

Rh is the correction factor for finite depth, h, of layer on a rigid base

(Fig. 3.3);

R is the correction factor for Poissons ratio of the soil, s (Fig. 3.4);Es is the Youngs modulus of elasticity of the soil; and

d is the diameter, or width, of the pile.


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Figure 3.1 Settlement-influence factor,I0.

Figure 3.2 Compressibility correction factor for settlement,RK.


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Figure 3.3 Depth correction factor for settlement,Rh.

Figure 3.4 Poissons ratio correction factor for settlement,R.

(R can also be approximated by:s

R +=

0.340.83 ).


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Table 3.1 Elastic parameters of various soils. (Source: Das, 1995.)

Type of Soil Es (MPa) s

Loose sand 10 25 0.20 0.40

Medium dense sand 17 28 0.25 0.40

Dense sand 35 55 0.30 0.45Silty sand 10 17 0.20 0.40

Sand and gravel 70 170 0.15 0.35

Soft clay 5 20 0.20 0.50

Medium clay 20 40 0.20 0.50

Stiff clay 40 100 0.20 0.50

3.2 End-Bearing Pile on Stiffer Stratum

The relationship given in Equation 3.1 is again used, except, in this case:

= RRRII bK0 (3.2)

where: Rb is the correction factor for stiffness of the bearing stratum (Fig. 3.5).

An important dimensionless parameter is the pile stiffness factor, K, where:

s

Ap

E

REK= (3.3)

where: Ep is the Youngs modulus of elasticity of the pile; and

RA is the area ratio of the pile, that is, the pile cross-sectional area divided

by its gross cross-sectional area (for a solid pile,RA = 1).

4. PILE GROUPS

Often pile foundations consist of two or more piles. Pile groups, as they are known, allow for

misalignments and other inadvertent eccentricities. Typical pile group configurations are shown in

Figure 4.1.

Piles in groups sometimes have a total load capacity greater than the sum of the load capacities of

the individual piles. However, in many cases, because the stress zones often overlap, as shown in

Figure 4.2, the total load capacity of the group is lower than the sum of the individual piles. This

notion can be quantified by the efficiency of the pile group, , such that:

= =

ultimate load capacity of group

sum of ultimate load capacities of individual piles

P

P

u group

u

( )(4.1)


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Figure 3.5 Base modulus correction factor for settlement,Rb.

An empirical relationship often used to quantify is the Converse-Labarre formula:

( ) ( ) =

+ 1

1 1

90

n m m n

m n(4.2)

where: ( )tan1 D s ;

n is the number of rows in the pile group;

m is the number of columns in the pile group;

D is the diameter of the piles in the group;

s is the spacing of the piles (see Figure 4.3).


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Figure 4.1 Typical pile group configurations. (Source: Bowles, 1996.)

Figure 4.2 Overlapping stress zones from adjacent piles. (Source: Das, 1995.)


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Figure 4.3 Pile group efficiency. (Source: Bowles, 1996.)

4.1 Bearing Capacity of Pile Groups

Piles in Sand

Das (1995) suggests that:

for driven piles with s 3D, Pu(group) may be taken to equal Pu of the individual piles, whichincludes the base resistance and the shaft adhesion;

for boredpiles with conventional spacings (s 3D), Pu(group) may be taken to equal 0.67 to 0.75times Pu of the individual piles.

Piles in Clay

Das (1995) suggests the following procedure:

1. Determine ( )P m n P Pu b s= + ;where: ( )P A cb b u b= 9 ( ) , where cu(b) is the undrained cohesion of the clay at the base of the pile;

and: P C c Ls u= .2. Determine the ultimate capacity of the pile group assuming that the group act as a block of

overall dimensions WBL. The shaft resistance of the block, Ps(block) is:

( )P W B c Ls group u( ) = +2 (4.3)

and the base resistance of the group, Pb(group) is:

P A c N b group b u b c( ) ( )= (4.4)

Values ofNc can be obtained from Figure 4.4.

3. The lower of the two values from 1 and 2 is the ultimate load capacity of the group.


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Figure 4.4 Values ofNc. (Source: Das, 1995.)

5. EXAMPLE

A four-storey building, 20 20 metres in plan, is proposed to be constructed on a site within thecity. The building is to be a steel frame with suspended concrete floor slabs. The design loads at

foundation level are 850, 1,700 and 3,400 kN. It is proposed to construct 900 mm diameter bored

pile with no enlarged base to a maximum length of around 20 m. Determine the number and length

of pile(s) needed to satisfy the design loads. The soil properties are given in Table 5.1.

Table 5.1 Soil profile and geotechnical properties.

Depth (m) Description c (kPa) E (MPa) s (kN/m3)

0 5 Clayey Sand, medium dense 5 35 10 0.3 20

5 35 Silty Clay, very stiff 150 0 10 0.3 18

Solution:

TryL = 20 metres.

cub

L

sbusuu NcAdzfCWPPP +=+=

0

(ignore W)

First, calculate the base resistance, Pbu:

Pbu =

=

0.9

4858.8 kN

2

150 9

Second, calculate the shaft resistance, Psu:

)claysilty()sandclayey( sususuPPP +=


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LCfP ssu =)sandclayey( (from Equation (2.3))

From Equation (2.7): 'tan' = vs Kf

From Table 2.1: KK0 = 1 sin and assume that = 0.5 (conservative assumption).

Therefore:( ) ( )

( ) ( ) kPa13.4350.5tan20535sin1

0.5tansin1tan'

==

== vvKf

Therefore: kN94.750.92

13.4)sandclayey(

==suP

LfCP ssu =)claysilty( and uus scf ==

From Figure 2.4, for c = su = 150 kPa, = 0.62, therefore: kPa930.62150 ==sf .

Therefore: Psu( )silty clay 0.9 3,944.3 kN= = 93 15

Hence: ( )P P Pu bu su= + = + + =858.8 94.7 3,944.3 4,897.8 kN

Therefore: PP

FSall

u= = =4,897.8

1,632.6 kN3

which is slightly less than 1,700 kN. Therefore

need to increase the length of the pile slightly. We require Pu = 1,700 3 = 5,100 kN. Assumingthat the base of the pile will remain in the silty clay layer, then:

( )P P P Pu bu su su= + = + + =858.8 94.7 5,100 kNsilty clay( ) .

Hence, we require: Psu( )silty clay 5,100 858.8 94.7 4,146.5 kN= =

But, LfCP ssu =)claysilty( , thus: L = =

4,146.5

0.915.8 m

93. Hence, we require a 900 mm

diameter pile of 15.8 + 5 = 20.8 metres in length.

850 kN pile:

We require Pu = 850 3 = 2,550 kN. Again, assuming that the base of the pile will remain in thesilty clay layer, then:

( )P P P Pu bu su su= + = + + =858.8 94.7 2silty clay( ) ,550 kN .

Hence, we require: Psu( )silty clay 2,550 858.8 94.7 1,596.5 kN= =

Thus: L =

=1,596.5

0.96.1 m

93. Hence, we require a 900 mm diameter pile of 6.1 + 5 = 11.1 m in

length.


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3,400 kN pile:

Try a pile group with 2 piles. Using the procedure for pile groups in clay.

1. ( )P mn P Pu b s= + = = 2 1 700, 3,400 kN .2. Using s = 3D = 3 0.9 = 2.7 m, the dimensions of the block are: WBL = 3.6 0.9 20.8.

( ) ( ) kN20,250151500.93.622)( =+=+= LcBWP ugroups

kN763.481504

0.92

)()( =

== cbubgroupb NcAP . (Nc from Fig. 4.4 with W/B = 3.6/0.9 = 4 and

L/B = 20.8/0.9 = 23.1). Therefore, Pu(group) = 20,250 + 763.4 = 21,013 kN.

Pall = Pu(group) / 3 = 7,005 kN >> 3,400 OK.

Therefore, use 2 20.8 m long, 900 mm diameter piles at a spacing of 2.7 metres.

6. PILE DESIGN DETAILS

6.1 Typical Bored Pile Diameters

Bored, cast in situ piles are generally available in the following diameters: 300, 450, 600, 750, 900,

1050, 1200 and 1500 mm. Other diameters may be available from individual contractors. The base

of bored piles can be under-reamed (belled) if the soil is suitable, that is, it does not collapse. Bells

with 60 angles are the most common, but 45 angles are also feasible, as shown in Figure 6.1.

Figure 6.1 Geometry of typical under-reamed (belled) piles. (Source: Fang, 1991.)

For the design of a pile with an enlarged base, all of the design equations presented previously are

still valid. When evaluating the base resistance of the pile, useBb

instead ofB. When evaluating


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the shaft resistance use Bs instead ofB. In addition, neglect the length of the bell, as the vertical

downward load is likely to separate the bell from the soil above.

6.2 Reinforcement Requirements for Concrete Piles

The Australian Standard for the design of piles, AS 21591995, specifies the followingreinforcement requirements:

The cross-sectional area of the longitudinal reinforcement,Ast, shall:

(a) be not less than 0.005Ag where a pile is fully embedded in the ground (where Ag is

the gross cross-sectional area of the pile);

(b) be not less than 0.01Ag for the portion of a pile projecting above the ground and the

limit may be reduced to 0.005Ag at a depth of 3 pile diameters below the ground; and

(c) not exceed 0.04Ag, unless it can be shown that the amount and disposition of the

reinforcement will not prevent the proper placing and compaction of the concrete.

In addition, AS 21591995 specifies that the minimum size of lateral reinforcement (ties and

helices) shall conform to Table 6.1.

Table 6.1 Bar sizes for ties and helices.

Pile Width (mm) Longitudinal Bar Diameter Minimum Diameter of Tie or Helix (mm)

Up to 500 Up to N28 5

Up to 500 N32 to N36 6501 to 700 all 6

701 and above all 10

7. UPLIFT CAPACITY OF PILES

In a number of situations, piles are required to carry tensile forces rather than the more common

compressive and/or lateral forces. These situations may include resistance to: uplift in buildings due

to hydrostatic pressures; expansive soil movements; and overturning due to wind, ice and stream

forces. In addition, tension piles are often used as reactions for guy ropes which stabilise antenna,mast and tower structures.

The ultimate tensile resistance of a pile, Ptu, is given by:

P P P W tu s buplift uplift = + + (7.1)

where: Psuplift is the shaft resistance from the several strata over the embedment

length, L, and is essentially the same as that obtained for piles in

compression. However, where a bell is used only empirical guidelines

are available (Fang, 1991). For shafts withL/Bs > 10, there is littleapparent increase in uplift resistance as a result of the bell. IfL/Bs

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